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Abstract—A voltage doubler rectifier for hostile environments, in particular at high temperatures, is presented. The system consists of a clamper section and a ...
2015 XVIII AISEM Annual Conference

Voltage doubler rectifier based on 4H-SiC diodes for high-temperatures energy harvesting applications Voltage doubler rectifier characterization up to T=300°C S. Rao, G. Pangallo and F.G. Della Corte

R. Nipoti

Department of Information Engineering, Infrastructures and Sustainable Energy, DIIES Università degli studi “Mediterranea” Reggio Calabria 89122, Italy

Institute for Microelectronics and Microsystems National Research Council CNR-IMM, UOS of Bologna Bologna 40129, Italy

Abstract—A voltage doubler rectifier for hostile environments, in particular at high temperatures, is presented. The system consists of a clamper section and a single diode rectifier working at higher temperatures with respect to the conventional operating thermal domain of silicon electronics. Both sections are realized with integrated 4H-SiC Schottky diodes. The rectified output amplitude signal voltage increases with the temperature due to the corresponding diode threshold voltage lowering. Keywords— High temperature devices; schottky diode; silicon carbide; voltage doubler rectifier; wide band gap semiconductors.

such as nuclear rich environments where the “conventional electronics” is generally not reliable enough. Other applications relate the fabrication of high temperature sensor [8,9,10] with high sensitivity and linear characteristics in a wide temperature range. A possible energy-efficient way to bias these sensors is a harvesting system based on piezoelectric materials [11]. For such sensing systems, however, an AC-DC rectifier operating at high-temperatures is needed. In this work, a voltage doubler rectifier for hostile environments, in particular at high temperatures, is presented.

I. INTRODUCTION The most common technology in the semiconductor industry is based on Silicon (Si). However, the physical properties of Si degrade when high thermal budgets are involved making Si-based devices not suitable for hostile or harsh environments. In this context, silicon carbide (SiC), gallium nitride (GaN) and other wide band-gap materials seem to be the most promising candidates for extreme applications. In particular, the high critical electric field (Ec=2-5 MV cm-1) [1] and the wide band-gap of 4H-SiC (Eg=3.23 eV) reducing the number of electron-hole pairs thermally generated in the material, make this semiconductor suitable for high voltage and high temperature sensing applications [2]. Moreover, SiC is characterized by a low intrinsic carrier concentration, high saturated electron velocity (vs=2×107 cm/s) and high thermal conductivity (σT=3–5 W/cm°C) allowing such devices to work in operating conditions more extreme than those of the Si counterpart.

II. DEVICE STRUCTURE The 4H-SiC Schottky diodes were fabricated at the institute for Microelectronics and Microsystems-CNR, Unit of Bologna (Italy). They have been fabricated on 7°62’ off-axis 4HSiC n-type, 300 μm-thick, homoepitaxial commercial wafer, with measured conductivity of 0.021 Ω×cm. The n-epilayer thickness is of 16.5 μm with a doping level of 3×1015 cm-3.

These favorable properties of SiC are desirable for efficient high power device operation where high current capability and elevated blocking voltages are required [3]. SiC is also an attractive material for high temperature operating gas sensors as well as solid-state transducers such as pressure sensors and accelerometers for automotive and space industry applications using microelectromechanical systems (MEMS) [4,5]. Moreover, many experimental studies demonstrated that 4H-SiC diodes are particularly suitable for the fabrication of UV sensors [6,7], used for flame detectors in very hostile areas

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Fig. 1. Schematic cross section of the 4H-SiC Schottky diode on a custom PCB.

2015 XVIII AISEM Annual Conference Sputter-deposition was exploited to deposit a 200nm-thick Ti/Al metal contact. Standard lithographic process and wet chemical etching were used to pattern the 150×150 µm2 Schottky contacts. Finally, a 200 nm-thick Ni film has been deposited on the n+ bulk to form the back contact. The chip was packaged and the contacts were bonded with aluminum wires, 50 µm in diameter, to a custom printed circuit board (PCB) on which the back contact of the diode was connected by means a silver conductive paint.

Reverse and forward I–V characteristics were measured using the Agilent 4155C Semiconductor Parameter Analyzer. The reverse current is about 1.43 µA at -50 V at a temperature of 25°C increasing up to ≈58 µA at 300°C as reported in Fig. 3 .

The schematic cross section of the vertical Schottky diode with the custom PCB is shown in Fig. 1.

III. DOUBLER VOLTAGE RECTIFIER The electrical circuit of the voltage doubler rectifier is shown in Fig.2. The circuit consists of two sections including, both, an integrated 4H-SiC Schottky diode. The first one, the clamper section, has an input capacitor, Cin, and a diode, D1, connected in series whereas, the second one is a standard single diode rectifier consisting of a diode D2 and a load resistance Rload [12]. Fig. 3. Reverse and forward I-V characteristics at room temperature and at T=300°C.

A 6V peak-to-peak sinusoidal signal at a frequency of 200 kHz has been applied at the rectifier input. In Fig.4 the rectifier and clamper output for Cin=47 nF and Rload=100 kΩ for two different temperatures (T=25 °C and T=300 °C) are shown. It can be observed that the rectifying device works correctly at temperatures of about 300 °C generally achieved in applications such as oil and gas exploration, nuclear rich environments and similar. It is worth noting that the output amplitude peak lightly increases with the temperature due to the corresponding diode threshold voltage decrease. Fig. 2. Doubler voltage rectifier

During the negative half-wave of the sinusoidal input signal, Cin is charged through D1 to the input peak voltage up to the diode threshold voltage Vγ. During the following positive half-wave, the AC input is moved up of the DC voltage previously stored into Cin. The AC input with the DC offset voltage is finally rectified by the second section of voltage doubler. The output voltage amplitude is therefore twice the input signal amplitude reduced of a factor 2×Vγ.

IV. RESULTS AND DISCUSSION The device has been tested in a thermostatic oven (Galli G210F030P) setting the reference temperature through its internal PID digital microcontroller in a range from 30°C up to 300°C.

978-1-4799-8591-3/15/$31.00 ©2015 IEEE

V. CONCLUSION A voltage doubler rectifier based on two integrated 4H-SiC Schottky diodes has been fabricated and characterized. It is particularly suitable for many applications where high operating temperatures and high voltage are required. The fabricated rectifier, working up to 300°C, is particularly useful, e.g., to harvest energy from piezoelectric materials or, more interesting, from radio frequency sources.

2015 XVIII AISEM Annual Conference REFERENCES [1]

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(b) Fig. 4. AC input (black waveform), clamper (red) and rectifier (blu) output voltage vs. time at T=25°C (a) and T=300°C (b).

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